1. Field of the Invention
The present invention relates to an ophthalmologic apparatus and a control method thereof, and more particularly to an ophthalmologic apparatus that evaluates a fundus image used for extracting a template image in the case where pattern matching of a fundus image is executed in detection of biological motion, and a control method thereof.
2. Description of the Related Art
In recent years, ophthalmologic images typified by fundus images have been used in medical practice as medical images for early detection of diseases. However, in medical practice using ophthalmologic images, the influence of eye movement during photographing has been recognized as a serious problem for long years. Specifically, eye movement may reduce clarity of ophthalmologic images to prevent an increase in accuracy of diagnosis or treatment. Human eyes are repeating involuntary slight vibration even during fixation on one point. This is referred to as involuntary eye movement. Since a patient cannot consciously stop involuntary eye movement, the influence of involuntary eye movement needs to be eliminated on the side of a medical practice tool or an operator in diagnosis or treatment of eyes.
For medical practice with high quality, it is important to eliminate involuntary eye movement without depending on operator's skills. For this purpose, a medical practice tool needs to have measures against eye movement. For example, as disclosed in a document “Daniel X. Hammer, R. Daniel Ferguson, John C. Magill, and Michael A. White, ‘Image stabilization for scanning laser ophthalmoscopy’, OPTICS EXPRESS 10(26), 1542-1549 (2002)”, a tracking technique of applying a light to optic disk in a circular motion, and detecting and correcting eye movement by variation of reflection.
However, the technique disclosed in the document troublesomely requires addition of hardware for detecting eye movement besides an ophthalmoscope. Thus, a tracking method that can be realized without additional hardware for detecting eye movement is suggested as disclosed in Japanese Patent Application Laid-Open No. H10-234674. This is a method of detecting movement of fundus in a lateral direction (a direction perpendicular to an eye depth direction) using a fundus observation apparatus, which is generally included in an ophthalmoscope for alignment, and executing tracking.
Also, as disclosed in Japanese Patent No. 04466968, a method is also proposed of scanning fundus to pick up tomographic images of fundus in a vertical direction (an eye depth direction) in positions shifted with time, and detecting eye movement using three-dimensional images synthesized from the tomographic images in a tomographic imaging apparatus.
Generally, in executing tracking, a small area having a certain characteristic is extracted from an entire fundus image, and movement of the small area between a plurality of continuous fundus images can be detected to detect movement of the entire fundus. With this method, a smaller amount of calculation is required than that in detecting movement of the entire fundus, thereby efficiently detecting movement of the fundus. An image of a small area having a certain characteristic used at this time is referred to as a template image, and a technique of pattern matching is used to detect movement of the template image. The pattern matching is a technique of searching an area most similar to the template image from the entire image to be referred to.
In detecting movement of fundus in pattern matching using a template image, a selected template image does not match an area to be detected or matches an area that is not to be detected in some cases, thereby causing false detection. Thus, a template image has to be properly selected. Also, in order to properly select a template image, it is important that a fundus image from which a template image is extracted is a proper image including no eye motion.
As described above, for the tracking technique by pattern matching, selection of a fundus image for extraction of a template image is very important. However, in the conventional method, whether a fundus image is a proper image is not determined in extraction of a template image. Thus, if pattern matching is not completed, a template image needs to be again extracted during imaging, which increases time and work for extracting a template image, and inevitably places burdens on an operator and a patient.
According to Japanese Patent Application Laid-Open No. H10-234674 above, a method is disclosed of selecting a plurality of small areas having a characteristic from a fundus image and executing matching on fundus images picked up thereafter to detect changes of the fundus images with time. However, determining whether a fundus image from which a small area is selected includes eye motion is not disclosed. Thus, it cannot be determined whether the small area is proper, and a small area needs to be again selected when matching is not completed, which increases burdens on an operator and a patient.
According to Japanese Patent No. 04466968, a technique is disclosed of synthesizing a plurality of tomographic images of fundus picked up in positions laterally shifted with time to once configure a three-dimensional image, and acquiring a lateral two-dimensional image of the fundus from the three-dimensional image to detect movement of the fundus. However, since a step is necessary of acquiring and synthesizing a plurality of tomographic images and further converting the images into a lateral image of the fundus, the movement cannot be detected at high speed during imaging. Also, Japanese Patent No. 04466968 does not disclose a technique of extracting a template image to execute tracking.